Chemical dispersants sprayed into the Gulf of Mexico to break up the massive oil spill from the Deepwater Horizon disaster do not appear to threaten the safety of seafood in the affected waters, the Food and Drug Administration said this week.

In a letter sent in response to questions from Rep. Edward J. Markey (D-Mass.), the agency responsible for ensuring the safety of seafood said that chemicals used to break up the slicks are not as dangerous to human health as the oil itself.

FDA scientists do not think that the chemicals accumulate significantly in the tissue of fish and shellfish; even if fish absorb the chemicals through gills or other ways, the fish do not retain them, Jeanne Ireland, the FDA's assistant commissioner for legislation, wrote to Markey. That means the chemicals are not passed up the food chain to humans and are not considered a public health concern, according to the FDA.

BP sprayed 1.8 million gallons of the dispersant Corexit on the surface of the gulf and, for the first time, at the wellhead a mile underwater. Dispersants were last used July 19, four days after BP temporarily capped its leaking well.

The ingredients in the dispersants were initially considered classified business information. Under a 1976 federal law, only a handful of employees at the Environmental Protection Agency were privy to the ingredients and were barred from divulging them.

But under pressure from the Obama administration, Corexit's manufacturer, Nalco Holding, agreed to disclose the ingredients to the EPA, which shared them with other federal and state agencies and then made them public.

The ingredients include propylene glycol, a chemical permitted by the FDA as a food additive and used in medicines, cosmetics and toothpaste; 2-butoxyethanol, which is found in cleaners, liquid soaps and cosmetics and quickly degrades in the environment; and a proprietary form of sulfonic acid salt, which is "moderately" toxic to freshwater fish and invertebrates but which the manufacturer says degrades quickly. In addition, Corexit contains volatile organic solvents that are made from crude oil and are not considered by the FDA to pose a public health concern because they do not accumulate significantly in the flesh of fish, according to Ireland.

The FDA is not monitoring fish and shellfish for the presence of Corexit in seafood because it is not considered a health risk, Ireland said.

Instead, the agency has examined gulf seafood for signs of oil contamination, which agency scientists say poses a health risk to anyone who eats the affected fish.

Sensory experts working for the FDA and NOAA smelled samples of gulf fish to make sure there was no odor from oil or chemicals. If the samples passed that test, they were subjected to laboratory analysis to detect polycyclic aromatic hydrocarbons, which come from oil. Some can cause cancer and other health effects in humans.

In consultation with the FDA, the National Oceanic and Atmospheric Administration has slowly reopened sections of gulf waters to fishing. About 24 percent of federal waters along the gulf are closed to fishing, down from a peak of about 37 percent in early June.

But Markey said questions remain about the impact of the chemicals over time. "We know almost nothing about the long-term effects of either oil or dispersants on the aquatic food chain," he said.